Carnival drumming

I'm one of those people who hide away during carnival and lament the loss of freedom to come and go as hundreds of thousands of revelers block the streets, beer cans in hand. But there's one thing about carnival that I love: the samba school drumming.

The feeling you get from the bleachers right in front of the drum section, which is where I was on Sunday, is that it's impossible to stand still. Instead of being considered just deafening, the deep, intense sound puts your brain almost into a trance. Curiously, this pleasure of carnival is due not to hearing, but to its neighbor, the much-despised sense of balance.

What keeps us upright is the ability to monitor the position of our head, thanks to vestibular motion sensors located in the labyrinth, inside the same bony box in your ears that contains the auditory cochlea. In addition to being neighbors, sound and head position sensors respond to the same stimulus: the mechanical deformation of tiny “hairs” in the receptor cells.

Normally, the sound intensities that move the “hair cells” of the auditory receptors do not even tickle the vestibular receptors, which have a different sensitivity range. Therefore, in everyday life, only the cochlea, and therefore hearing, is responsible for sounds.

But everything changes in front of a samba school drum section (or orchestra, funk dance party, or your favorite rave). As Neil Todd, a researcher at the University of Manchester in the United Kingdom, has shown, low-frequency sounds (below 200 Hz) that are loud enough (above 90 dB, comparable to a drill or truck on the road), such as the beat of the surdos and caixas, can stimulate both types of receptors, auditory and vestibular. The result: your brain receives the signal that you are moving to the beat of the music. From there, it's a short step to actually moving to the beat of the music, incorporating vestibular activity. The drums really shake your body.

And it's still enjoyable—that is, for those who like vestibular stimulation, such as swings and roller coasters. Up to 90 dB, louder bass sounds are just increasingly irritating—but as soon as they begin to stimulate the vestibular sense, bass sounds become enjoyable again (while treble sounds only become more unpleasant). That's why you can't listen to baticum—or any music full of bass—at low volume. It's either loud or it has no effect.